ANN ARBOR—Many people assume that the more testosterone, the better, but a new University of Michigan study finds that might not always be the case in romantic relationships.

Low testosterone levels may be a good thing for both men and women, who reported more satisfaction and commitment to their relationships when they had lower levels.

Higher testosterone is generally thought to be associated with attracting sexual partners, but might not be compatible with some kinds of long-term relationships, said Robin Edelstein, U-M associate professor of psychology and the study's lead author.

"The assumption is generally that high testosterone is good for sexual relationships," she said. "These findings suggest that once people are in a relationship, lower levels of testosterone may be beneficial—or may reflect better ongoing relationship dynamics."

Previous studies have examined how testosterone levels are associated with the quality of men's relationships, but the U-M research is among the first to demonstrate the association among women.

The study used data from 39 heterosexual couples whose ages ranged from 18 to 31 and were in relationships from two months to seven years. They answered questions about their satisfaction ("My relationship is close to ideal"), commitment ("I want our relationship to last forever") and investment ("I have invested a great deal into our relationship that I would lose if the relationship were to end"). Participants also provided their saliva for analysis.

Researchers concluded that the quality of a person's relationship was associated with his/her own and his/her partner's testosterone levels. Both men's and women's testosterone was negatively correlated with their own and their partner's satisfaction and commitment. The couples were more satisfied and committed when they or their partner had low testosterone levels, the research indicated.

The findings appear in the April issue of Hormones and Behavior.

The study's other authors include Sari van Anders, William Chopik, Katherine Goldey and Britney Wardecker.